FIG. 1 shows a very schematic, known in the industry, exemplary drive train of a motor vehicle which is designed as a hybrid vehicle, whereby such a hybrid vehicle comprises a drive aggregate 1, designed as hybrid drive, which is formed having a combustion engine 2 and an electric machine 3. A transmission 5 is positioned between the drive aggregate 1 and an output 4. The transmission 5 represents here an especially automatic or automated shift transmission. The transmission 5 comprises of an amount N of shift elements 9, wherein FIG. 1 shows five shift elements 9. A first defined partial amount M of the shift elements 9 is engaged in each gear and a second defined partial amount N-M of the shift elements 9 are disengaged. Thus, it can be provided in FIG. 1 that in each engaged gear three of the 5 shift elements 9 are engaged and two of the five shift elements 9 are disengaged.
FIG. 1 also shows a clutch 6 provided between the combustion engine 2 and the electric machine 3 of the drive aggregate 1, wherein the combustion engine 2 is decoupled from the output 4 when the clutch 6 is disengaged and is then shutdown so as to operate the hybrid vehicle, in electric mode, exclusively by the electric machine 3 of the drive aggregate. To the contrary, when the clutch 6 is engaged, the combustion engine 2 is coupled to the output 4 and the hybrid vehicle can be driven by the combustion engine 2 as well as by the electric machine 3 or rather by both simultaneously.
An electric energy storage 7 operates together with the electric machine 3, in accordance with FIG. 1, which in the motor operation of the electric machine 3 of the drive aggregate 1 is more discharged, and in generator operation of the electric machine 3 of the drive aggregate 1 is more charged. A brake 13, in accordance with FIG. 1, operates together with the output 4.
The drive train of FIG. 1 has in addition a hydraulic system which comprises a main pump 8 which, beginning with the drive aggregate 1, can be mechanically driven. At the time when, for instance, during electric drive at low speed with a disengaged clutch 6, the rotational speed of the electric machine 3 of the drive aggregate 1, which drives the main pump 8, is too low, it is possible that the main pump 8 cannot respond to a hydraulic pressure request. It is already known in practice for the hydraulic system of the motor vehicle, in addition to the main pump 8, to be driven by an electric auxiliary pump 10, independent from the drive aggregate 1, by a separate electric motor 11 of the auxiliary pump 10.
FIG. 1 also shows a control device 12 which exchanges data with the transmission 5 and the electric motor 11 of the auxiliary pump 10, as illustrated by double-arrows, for controlling and/or adjusting the operation of the electric machine 3. In addition, the control device 12 exchanges data with the brake 13, an accelerator pedal 14, and a brake pedal 15 of the motor vehicle. The control device 12 can be a hybrid control device in a hybrid vehicle or also a transmission control device.
To monitor the auxiliary pump 10 and to guarantee functioning of the auxiliary pump 10, in motor vehicles known in the practice, a pressure sensor is installed in the transmission 5 to test the level of hydraulic pressure provided by the auxiliary pump 10. Such a pressure sensor requires installation space and causes additional cost.
DE 10 2008 040 667 A1 teaches a hydraulic system of a motor vehicle with a main pump and an auxiliary pump, as well as details a method for operating this hydraulic pressure system.
There is a requirement for a method for operating a motor vehicle and a control device for a motor vehicle, through which a pressure sensor for the functional test of the auxiliary pump of the hydraulic system can be omitted.